Pressure-sensitive adhesive composition, pressure-sensitive adhesive sheet, and method for producing the same

ABSTRACT

The present invention relates to a pressure-sensitive adhesive composition including, as a main ingredient, a monomer mixture or an acrylic copolymer material obtained by at least partially polymerizing the monomer mixture, the monomer mixture including: at least one monomer (monomer m1) selected from alkyl (meth)acrylates represented by the following formula (I) CH 2 ═C(R 1 )COOR 2  in which R 1  is a hydrogen atom or a methyl group, and R 2  is an alkyl group having 1 to 20 carbon atoms: at least one monomer (monomer m2) selected from N-hydroxyalkyl(meth)acrylamides represented by the following formula (II) CH 2 ═C(R 1 )CONHR 4  in which R 3  is a hydrogen atom or a methyl group, and R 4  is a hydroxyalkyl group having 2 to 4 carbon atoms: and at least one monomer (monomer m3) selected from N-vinyl cyclic amides and (meth)acrylamides that may have an N-alkyl group; and the monomer mixture including substantially no carboxyl group-containing monomer.

FIELD OF THE INVENTION

The present invention relates to a pressure-sensitive adhesive(hereinafter also referred to as adhesive, and the same shall applyhereinafter) composition and a pressure-sensitive adhesive sheet, andspecifically, it relates to a pressure-sensitive adhesive sheet suitablefor the applications for directly attaching to a metal surface or forother various applications, and relates to a pressure-sensitive adhesivecomposition for the pressure-sensitive adhesive sheet. The presentinvention further relates to a method for producing thepressure-sensitive adhesive sheet.

BACKGROUND OF THE INVENTION

Recently, the pressure-sensitive adhesive sheets have been employed invarious aspects such as fixation (bonding), conveyance, protection,decoration, and the like of articles. Typical examples of thepressure-sensitive adhesive sheets include those provided with apressure-sensitive adhesive layer formed by using an acrylic adhesivecomposition. This acrylic adhesive composition is configured such thatit may form a pressure-sensitive adhesive layer containing an acryliccopolymer as a base polymer. As the acrylic copolymer, those containingalkyl (meth)acrylate as a main ingredient (a main component) and furthercontaining a copolymerization composition containing a carboxylgroup-containing monomers such as acrylic acid, for the purpose ofimproving adhesive performance, and the like are generally used.

However, for a pressure-sensitive adhesive sheet used for the articleshaving a part composed of a material that is susceptible to an acid (forexample, an article having a metal surface, such as electronic parts),when it has the copolymerization composition containing a carboxylgroup-containing monomer as described above, the carboxyl group in theacrylic copolymer may be responsible for the unfavorable effects on theabove-described material (for example, the corrosion of the metalsurface). Accordingly, the pressure-sensitive adhesive composition forforming a pressure-sensitive adhesive layer provided in apressure-sensitive adhesive sheet for these applications (particularly,an application for directly attaching to a metal surface) is preferablyin a constitution enabling the formation of a pressure-sensitiveadhesive layer having, as a base polymer, an acrylic copolymer having acopolymerization composition containing no carboxyl group-containingmonomer. As the background art documents regarding these kinds oftechnologies, JP-A-2007-63536, JP-A-2005-325250 and JP-A-2007-264092 maybe exemplified. As the background art documents regarding the acrylicadhesive, JP-A-2007-264092 may be exemplified. JP-A-2003-165965 is abackground art document regarding a pressure-sensitive adhesive and anadhesive.

However, a pressure-sensitive adhesive composition containing nocarboxyl group-containing monomer tends to have insufficient adhesioncharacteristics of a pressure-sensitive adhesive sheet formed from thecomposition. Regarding this, JP-A-2007-63536 describes copolymerizationwith a specific maleimide-based compound, JP-A-2005-325250 describescopolymerization with a nitrogen atom-containing monomers such asN-acryloylmorpholine, and JP-A-2000-303045 describes use of specificmonomers such as phenoxyethyl acrylate, as main monomer components.However, even in the pressure-sensitive adhesive sheets formed by usingthese technologies, there is room for improvement in adhesiveperformance. For example, it would be useful to provide apressure-sensitive adhesive composition that exhibits a desired cohesiveforce without the use of a carboxyl group-containing monomer, andfurther is capable of forming a pressure-sensitive adhesive sheet havingincreased repelling resistance (adhesiveness on a curved surface).

SUMMARY OF THE INVENTION

The present invention has been made in light of the conventionalcircumstances, and an object of the invention is to provide an acrylicadhesive composition that enables formation of a pressure-sensitiveadhesive sheet having improved adhesive performances (particularlyrepelling resistance), without the substantial use of a carboxylgroup-containing monomer. Another object of the present invention is toprovide a pressure-sensitive adhesive sheet using the pressure-sensitiveadhesive composition. A further object of the present invention is toprovide a method for producing the pressure-sensitive adhesive sheet.

Namely, the present invention relates to the following items (1) to (8).

-   (1) A pressure-sensitive adhesive composition including, as a main    ingredient, a monomer mixture or an acrylic copolymer material    obtained by at least partially polymerizing the monomer mixture,

the monomer mixture including:

at least one monomer (monomer m1) selected from alkyl (meth)acrylatesrepresented by the following formula (I) in an amount of 50 to 85% byweight based on a total amount of the monomer mixture:CH₂═C(R¹)COOR²  (I)in which R¹ is a hydrogen atom or a methyl group, and R² is an alkylgroup having 1 to 20 carbon atoms,

at least one monomer (monomer m2) selected fromN-hydroxyalkyl(meth)acrylamides represented by the following formula(II) in an amount of 0.1 to 12% by weight based on the total amount ofthe monomer mixture:CH₂═C(R³)CONHR⁴  (II)in which R³ is a hydrogen atom or a methyl group, and R⁴ is ahydroxyalkyl group having 2 to 4 carbon atoms, and

at least one monomer (monomer m3) selected from N-vinyl cyclic amidesand (meth)acrylamides that may have an N-alkyl group, in an amount of 10to 40% by weight based on the total amount of the monomer mixture; and

the monomer mixture including substantially no carboxyl group-containingmonomer.

-   (2) The composition according to (1), in which the monomer m1, the    monomer m2 and the monomer m3 are contained in a total amount of 90%    by weight or more based on the total amount of the monomer mixture.-   (3) The composition according to (1) or (2), in which the monomer    mixture has such a monomer composition that the acrylic copolymer,    obtained by polymerizing the monomer mixture, has a glass transition    temperature (Tg) of −10° C. or lower.-   (4) The composition according to any one of (1) to (3), in which the    monomer m2 is N-(2-hydroxyethyl)(meth)acrylamide.-   (5) The composition according to any one of (1) to (4), in which the    monomer m3 is at least one monomer selected from N-vinyl cyclic    amides represented by the following formula (III):

in which R⁵ is a divalent organic group.

-   (6) The composition according to any one of (1) to (5), which is    used for a pressure-sensitive adhesive sheet to be directly attached    to a metal surface.-   (7) A pressure-sensitive adhesive sheet including a    pressure-sensitive adhesive layer formed by using the composition    according to any one of (1) to (6).-   (8) A method for producing a pressure-sensitive adhesive sheet, the    method including:

preparing a pressure-sensitive adhesive composition including a monomermixture or an acrylic copolymer material obtained by partiallypolymerizing the monomer mixture, and 0.01 to 2 parts by weight of aphotopolymerization initiator based on 100 parts by weight of themonomer mixture,

the monomer mixture including:

at least one monomer (monomer m1) selected from alkyl (meth)acrylatesrepresented by the following formula (I) in an amount of 50 to 85% byweight based on the total amount of the monomer mixture:CH₂═C(R¹)COOR²  (I)in which R¹ is a hydrogen atom or a methyl group, and R² is an alkylgroup having 1 to 20 carbon atoms,

at least one monomer (monomer m2) selected fromN-hydroxyalkyl(meth)acrylamides represented by the following formula(II) in an amount of 0.1 to 12% by weight based on the total amount ofthe monomer mixture:CH₂═C(R³)CONHR⁴  (II)in which R³ is a hydrogen atom or a methyl group, and R⁴ is ahydroxyalkyl group having 2 to 4 carbon atoms, and

at least one monomer (monomer m3) selected from N-vinyl cyclic amidesand (meth)acrylamides that may have an N-alkyl group, in an amount of 10to 40% by weight based on the total amount of the monomer mixture, and

the monomer mixture including substantially no carboxyl group-containingmonomer;

applying the pressure-sensitive adhesive composition to a support; and

curing the applied composition by irradiating the applied compositionwith light to thereby form a pressure-sensitive adhesive layer.

The “acrylic copolymer material” may be a partially polymerized productof the monomer mixture, or an acrylic copolymer obtained bypolymerization of approximately all of the monomer mixture (for example,at a conversion of about 95% or more, preferably about 99% or more). Thepartially polymerized product of the monomer mixture can typicallyinclude a polymer obtained by polymerizing a partial proportion ofmonomers constituting the monomer mixture (it may include a monomerhaving a relatively low polymerization degree, and for example, it mayinclude a polymer having a weight average molecular weight of usually nomore than 1×10⁴ (which may also be referred to as oligomer)), andunpolymerized monomers. A conversion of the partially polymerizedproduct (that is, a ratio of the monomers that have been polymerized inadvance in the used monomers) may be, for example, about 2 to 50%.Further, the expression as disclosed herein that the pressure-sensitiveadhesive composition include, as main ingredients, the monomer mixtureor the acrylic copolymer material, means that the acrylic copolymerobtained by polymerizing the monomer mixture (which may be a polymerobtained by the polymerization or the curing of the monomer mixture or apartially polymerized product thereof by irradiating light, or the like,or substantially the acrylic copolymer material per se) constitutes, asa base polymer (a main component in the polymer component), apressure-sensitive adhesive formed with the pressure-sensitive adhesivecomposition.

Since the pressure-sensitive adhesive composition contains, as mainingredients, the monomer mixture or an acrylic copolymer material thatdoes not substantially employ a monomer containing a carboxyl group (acarboxyl group-containing monomer), even when a pressure-sensitiveadhesive layer formed by using the composition is directly attached on asurface composed of a material that is susceptible to an acid, such as ametal surface, the corrosion of the metal surface, and the like by thecarboxyl group can be prevented. In addition, by using, as mainingredients, an acrylic copolymer having a monomer composition (aproportional ratio of each of the monomers used as a monomer component)including the monomer m1 as a main component together with predeterminedamounts of the monomer m2 and the monomer m3, a pressure-sensitiveadhesive layer (or consequently, a pressure-sensitive adhesive sheetincluding the pressure-sensitive adhesive layer, and this shall applyhereinafter) having a high cohesive force, and exhibiting furtherimproved repelling resistance can be formed.

As a preferred example of the monomer m2,N-(2-hydroxyethyl)(meth)acrylamides can be mentioned. Among these,N-(2-hydroxyethyl)acrylamide is preferably used. Further, as the monomerm3, at least one monomer selected from N-vinyl cyclic amides representedby the following formula (III) can be preferably used:

in which R⁵ is a divalent organic group.

As a preferred example of the N-vinyl cyclic amide,N-vinyl-2-pyrrolidone (i.e., a compound in which R⁵ in the formula (III)is a propylene group) can be mentioned.

In a preferred embodiment of the pressure-sensitive adhesive compositionas disclosed here, the total amount of the monomers m1, m2 and m3 isabout 90% by weight or more based on the total amount of monomermixture. The pressure-sensitive adhesive composition contains, as mainingredients, the monomer mixture of such a composition or an acryliccopolymer material can form a pressure-sensitive adhesive layer havingmore excellent quality stability.

The monomer mixture preferably has such a monomer composition that theacrylic copolymer obtained by polymerizing the monomer mixture (that is,a base polymer in a pressure-sensitive adhesive formed with thepressure-sensitive adhesive composition) has a glass transitiontemperature (Tg) of about −10° C. or lower (typically about −10 to −70°C.). This adhesive composition can form a pressure-sensitive adhesivelayer exhibiting good adhesive performances (tackiness, and the like),for example, at a normal temperature (typically around 5 to 35° C., forexample, 20 to 25° C.).

In another preferred embodiment of the pressure-sensitive adhesivecomposition as disclosed herein, the composition contains aphotopolymerization initiator, and is composed of a pressure-sensitiveadhesive composition that is curable by irradiation of light (forexample, an ultraviolet ray) (a photocurable pressure-sensitive adhesivecomposition). For example, it may be a pressure-sensitive adhesivecomposition formed by using about 0.01 to 2 parts by weight of aphotopolymerization initiator, based on 100 parts by weight of themonomer mixture. A pressure-sensitive adhesive layer formed byirradiation of light onto this photocurable pressure-sensitive adhesivecomposition (which may be referred to as a photopolymerizablepressure-sensitive adhesive composition, or the like) may exhibit betteradhesive performances (for example, a cohesive force, repellingresistance, and the like).

According to the present invention, a pressure-sensitive adhesive sheetwhich includes a pressure-sensitive adhesive layer containing, as a basepolymer, an acrylic copolymer obtained by polymerizing any one of themonomer mixture as disclosed herein is provided. The pressure-sensitiveadhesive layer can be a pressure-sensitive adhesive layer formed byusing any one of the pressure-sensitive adhesive compositions asdisclosed herein. That is, according to the present invention, apressure-sensitive adhesive sheet including a pressure-sensitiveadhesive layer formed by using any one of the pressure-sensitiveadhesive compositions as disclosed herein is provided. Since thispressure-sensitive adhesive sheet contains, as a base polymer, a polymerderived from the monomer mixture or an acrylic copolymer material, evenwhen it is directly attached on a surface composed of a material that issusceptible to an acid, such as a metal surface, there is no corrosionof the metal surface, and the like. Accordingly, it is suitable as apressure-sensitive adhesive sheet used for directly attaching to a metalsurface (a pressure-sensitive adhesive sheet to be attached to a metalsurface). Further, the pressure-sensitive adhesive sheet can exhibitgood adhesiveness on an object to be adhered other than the metalsurface, as well as excellent cohesive force and repelling resistance.Moreover, any one of the pressure-sensitive adhesive compositions asdisclosed herein is suitable as a pressure-sensitive adhesivecomposition used for forming a pressure-sensitive adhesive layer of apressure-sensitive adhesive sheet to be attached to a metal surface.

According to the present invention, a method for producing apressure-sensitive adhesive sheet on a metal surface (which can be apressure-sensitive adhesive sheet to be attached to a metal surface) isalso provided. The method includes a step of preparing apressure-sensitive adhesive composition including the monomer mixturesatisfying all of the following conditions (a) to (d), or an acryliccopolymer material obtained by partial polymerization of the monomermixture. In a preferred embodiment, in this step, a photopolymerizationinitiator is used in an amount of about 0.01 to 2 parts by weight basedon 100 parts by weight of the monomer mixture.

(a) The monomer mixture contains at least one monomer (monomer m1)selected from alkyl (meth)acrylates represented by the following formula(I). The content of the monomer m1 may be from 50 to 85% by weight basedon the total amount of monomer mixture:CH₂═C(R¹)COOR²  (I)in which R¹ is a hydrogen atom or a methyl group, and R² is an alkylgroup having 1 to 20 carbon atoms.

(b) The monomer mixture contains at lease one monomer (monomer m2)selected from N-hydroxyalkyl(meth)acrylamides represented by thefollowing formula (II). The content of the monomer m2 may be from 0.1 to12% by weight based on the total amount of monomer mixture:CH₂═C(R³)CONHR⁴  (II)in which R³ is a hydrogen atom or a methyl group, and R⁴ is ahydroxyalkyl group having 2 to 4 carbon atoms.

(c) The monomer mixture contains at least one monomer (monomer m3)selected from N-vinyl cyclic amide and (meth)acrylamide that may have anN-alkyl group. The content of the monomer m3 may be from 10 to 40% byweight based on the total monomer mixture.

(d) The monomer mixture includes substantially no carboxylgroup-containing monomer.

The method for producing the pressure-sensitive adhesive sheet furtherincludes a step of applying the pressure-sensitive adhesive compositiononto a support, and a step of curing the composition by irradiation oflight onto the applied composition to thereby form a pressure-sensitiveadhesive layer.

By this method, a pressure-sensitive adhesive sheet which can bepreferably used even for a surface that is susceptible to an acid, suchas a metal surface, and further has good adhesive performances (cohesiveforce, repelling resistance, and the like) can be efficiently prepared.Further, the method for producing the pressure-sensitive adhesive sheetin which a pressure-sensitive adhesive layer is formed by using thephotocurable adhesive composition as described above is preferred inthat it can easily form a thick adhesive layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing a configurationexample of the pressure-sensitive adhesive sheet according to thepresent invention.

FIG. 2 is another schematic cross-sectional view showing theconfiguration example of the pressure-sensitive adhesive sheet accordingto the present invention.

FIG. 3 is a still another schematic cross-sectional view showing theconfiguration example of the pressure-sensitive adhesive sheet accordingto the present invention.

FIG. 4 is a further schematic cross-sectional view showing theconfiguration example of the pressure-sensitive adhesive sheet accordingto the present invention.

FIG. 5 is a still further schematic cross-sectional view showing theconfiguration example of the pressure-sensitive adhesive sheet accordingto the present invention.

FIG. 6 is a still more further schematic cross-sectional view showingthe configuration example of the pressure-sensitive adhesive sheetaccording to the present invention.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

-   1: Substrate-   2: Pressure-sensitive adhesive layer-   3: Release liner-   11, 12, 13, 14, 15, and 16: Pressure-sensitive adhesive sheets

DETAILED DESCRIPTION OF THE INVENTION

Hereinbelow, suitable embodiments of the present invention will bedescribed. Further, the factors required to carry out the presentinvention other than the factors specifically mentioned in the presentspecification can be figured out by the design of a skilled person inthe art on the basis of a background art in the relevant field. Thepresent invention can be carried out in accordance with a commontechnological knowledge in the relevant field and the contents disclosedin the present specification.

The pressure-sensitive adhesive composition as disclosed herein has, asmain ingredients, predetermined monomer mixture, or an acrylic copolymermaterial obtained by partial polymerization of the monomer mixture. Themonomer mixture includes at least the monomer m1, the monomer m2, andthe monomer m3 as essential components.

The monomer m1 is a component which is a main monomer (main component)constituting the monomer mixture, and includes alkyl (meth)acrylaterepresented by the following formula (I), that is, (meth)acrylic esterof alkyl alcohol. Here, the “(meth)acrylic acid” is meant to encompassacrylic acid and methacrylic acid. Further, the expression that themonomer m1 is a main monomer means that the amount of the monomer m1 (ina case of including two or more alkyl (meth)acrylates represented by theformula (I), the total amount thereof) is 50% by weight or more based onthe total amount of the monomer mixture.

The monomer m1 can be one or more selected from alkyl (meth)acrylatesrepresented by the following formula (I).CH₂═C(R¹)COOR²  (I)

Here, R¹ in the formula (I) is a hydrogen atom or a methyl group.Further, R² in the formula (I) is an alkyl group having 1 to 20 carbonatoms. The alkyl group may be linear or branched. Specific examples ofalkyl (meth)acrylate represented by the formula (I) include methyl(meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isobutyl(meth)acrylate, sec-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl(meth)acrylate, isopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl(meth)acrylate, n-octyl (meth)acrylate, isooctyl (meth)acrylate,2-ethylhexyl (meth)acrylate, nonyl (meth)acrylate, isononyl(meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl(meth)acrylate, dodecyl (meth)acrylate, tridecyl (meth)acrylate,tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl(meth)acrylate, heptadecyl (meth)acrylate, octadecyl (meth)acrylate,nonadecyl (meth)acrylate and eicosyl (meth)acrylate. Among these, alkyl(meth)acrylate in which R² is an alkyl group having 2 to 14 carbon atoms(the range of carbon atoms may be hereinafter referred to “C₂₋₁₄”) ispreferable, and alkyl (meth)acrylate in which R² is a C₂₋₁₀ alkyl group(for example, an n-butyl group, a 2-ethylhexyl group, and the like) ismore preferable.

In a preferred embodiment, the alkyl (meth)acrylate in which R² in theformula (1) is a C₂₋₁₀ (more preferably C₄₋₈) alkyl group occupies about70% by weight or more (more preferably about 90% by weight or more) ofthe total amount of the monomer m1. Substantially all of the monomer m1may be C₂₋₁₀ alkyl (more preferably C₄₋₈ alkyl) (meth)acrylate. Themonomer mixture may be, for example, a composition including butylacrylate (BA) alone, a composition including 2-ethylhexyl acrylate(2EHA) alone, a composition including the two kinds, that is, BA and2EHA, as a monomer m1.

The amount of the monomer m1 can be about from 50 to 85% by weight(preferably 60 to 80% by weight) based on the total amount of themonomer mixture. When the amount of the monomer m1 is lower than theabove-described range, the pressure-sensitive adhesive layer formed fromthe composition has a tendency of having insufficient adhesiveperformances (adhesiveness, tackiness, and the like). On the other hand,when the amount of the monomer m1 is higher than the above-describedrange, since the amount of the monomer m2 and the monomer m3 containedin the monomer mixture is decreased, the compatibility between thecohesive force (for example, durability against peeling under a certainstress, that is a static load characteristic) and the repellingresistance may be deteriorated. Further, it is usual that thecomposition of the monomer mixture (the monomer composition) correspondsto the copolymerization ratio of an acrylic copolymer obtained bypolymerization of the monomer mixture (that is, a base polymer in apressure-sensitive adhesive formed from the pressure-sensitive adhesivecomposition as disclosed herein).

The monomer mixture further includes the monomer m2, in addition to themonomer m1 as a main monomer. This monomer m2 may be one or moreselected from N-hydroxyalkyl (meth)acrylamides represented by thefollowing formula (II).CH₂∇C(R³)CONHR⁴  (II)

Here, R³ in the formula (II) is a hydrogen atom or a methyl group.Further, R⁴ in the formula (II) is a hydroxyalkyl group having 2 to 4carbon atoms. The alkyl group in the hydroxyalkyl group may be linear orbranched. Specific examples of N-hydroxyalkyl(meth)acrylamiderepresented by the formula (II) include N-(2-hydroxyethyl)acrylamide,N-(2-hydroxyethyl)methacrylamide, N-(2-hydroxypropyl)acrylamide,N-(2-hydroxypropyl)methacrylamide, N-(1-hydroxypropyl)acrylamide,N-(1-hydroxypropyl)methacrylamide, N-(3-hydroxypropyl)acrylamide,N-(3-hydroxypropyl)methacrylamide, N-(2-hydroxybutyl)acrylamide,N-(2-hydroxybutyl)methacrylamide, N-(3-hydroxybutyl)acrylamide,N-(3-hydroxybutyl)methacrylamide, N-(4-hydroxybutyl)acrylamide andN-(4-hydroxybutyl)methacrylamide. Examples of the monomer m2 in thepresent invention, that is preferable from the viewpoint that apressure-sensitive adhesive layer having good balance betweenhydrophilicity and hydrophobicity, and excellent balance among thepressure-sensitive adhesive properties can be formed, includeN-(2-hydroxyethyl)acrylamide and N-(2-hydroxyethyl)methacrylamide.Particularly, N-(2-hydroxyethyl) acrylamide (HEAA) is preferably used.For example, 50% by weight or more (more preferably 70% by weight ormore, typically substantially all) of the monomer m2 is preferably HEAA.

The monomer m2 can function as a component contributing to the improvedaggregation of a pressure-sensitive adhesive due to the interactionbetween the molecules of the monomer m2. The amount of the monomer m2may be about 0.1 to 12% by weight (typically about 1 to 10% by weight)based on the total amount of the monomer mixture. When the amount of themonomer m2 is lower than the above-described range, thepressure-sensitive adhesive layer formed from the composition has atendency of having insufficient adhesive performances (adhesiveness at ahigh temperature, durability against peeling under a certain stress, andthe like). On the other hand, when the amount of the monomer m2 ishigher than the above-described range, there is a tendency thattackiness or the adhesiveness at a lower temperature is lowered.

In a preferred embodiment of the pressure-sensitive adhesive compositionas disclosed herein, the amount of the monomer m2 is about 2% by weightor more (typically 2 to 12% by weight), and more preferably about 3% byweight or more (typically about 3 to 12% by weight, for example about 3to 10% by weight) based on the total amount of the monomer mixture. Bythis adhesive composition, an effect that a pressure-sensitive adhesivesheet having better cohesive force and repelling resistance can beformed can be obtained.

The weight ratio of the monomer m1 and the monomer m2 in the monomermixture (m1/m2) can be, for example, about 99/1 to 80/20. Usually, theweight ratio is preferably about 98/2 to 85/15 (more preferably about97/3 to 90/10). By this adhesive composition, an effect that apressure-sensitive adhesive sheet having better cohesive force andrepelling resistance can be formed can be obtained. The total amount ofthe monomer m1 and the monomer m2 may be, for example, about 60 to 90%by weight based on the total amount of the monomer mixture. The totalamount thereof is preferably about 70% by weight or more (for example,70 to 90% by weight) based on the total amount of the monomer mixture.

The monomer mixture further includes a monomer m3, in addition to themonomers m1 and m2. This monomer m3 may be at least one monomer selectedfrom the group consisting of N-vinyl cyclic amide; and (meth)acrylamidethat may have an N-alkyl group. Specific examples of N-vinyl cyclicamide include N-vinyl-2-pyrrolidone, N-vinyl-2-piperidone,N-vinyl-3-morpholinone, N-vinyl-2-caprolactam, N-vinyl-1,3-oxadine-2-oneand N-vinyl-3,5-morpholinedione. Specific examples of (meth)acrylamidethat may have an N-alkyl group include (meth)acrylamides such asN-alkyl(meth)acrylamides; N-ethyl(meth)acrylamide,N-n-butyl(meth)acrylamide; and N,N-dialkyl(meth)acrylamides such asN,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide,N,N-dipropyl(meth)acrylamide, N,N-diisopropyl(meth)acrylamide,N,N-di(n-butyl)(meth)acrylamide, N,N-di(t-butyl)(meth)acrylamide.(Meth)acrylamide containing one or two N-alkyl groups having 1 to 4carbon atoms (more preferably 1 or 2 carbon atoms) (for example,N,N-dialkylacrylamides such as N,N-diethylacrylamide,N,N-dimethylacrylamide) is preferably used.

As the monomer m3 in the technology as disclosed herein, N-vinyl cyclicamide represented by the following formula (III) can be preferably used:

in which R⁵ is a divalent organic group. R⁵ in the above formula ispreferably a saturated or unsaturated hydrocarbon group, more preferablya saturated hydrocarbon group (for example, an alkylene group having 3to 5 carbon atoms). By the monomer mixture having such a composition, apressure-sensitive adhesive composition for forming a pressure-sensitiveadhesive layer having good balance among the adhesion characteristicscan be realized. In the present invention, N-vinyl-2-pyrrolidone isexemplified as a particularly preferred N-vinyl cyclic amide.

The amount of the monomer m3 may be, for example, about 10 to 40% byweight based on the total amount of the monomer mixture. When the amountof the monomer m3 is higher than the above-described range, thecharacteristics of a pressure-sensitive adhesive layer formed with thecomposition under an environment at a low temperature (the adhesivenessas a pressure-sensitive adhesive, and the like) may be deteriorated. Onthe other hand, when the amount of the monomer m3 is lower than theabove-described range, the adhesion force (for example, the peelingstrength) or the repelling resistance tends to be lowered. By settingthe amount of the monomer m3 of about 15 to 35% by weight based on thetotal amount of the monomer mixture, a more suitable result can berealized.

The total amount of the monomers m1, m2, and m3 can be, for example,about 70% by weight or more based on the total amount of the monomermixture. The total amount thereof is preferably about 90% by weight ormore based on the total amount of the monomer mixture (more preferablyabout 95% by weight or more). In a preferred embodiment of thepressure-sensitive adhesive composition as disclosed herein, the monomermixture is substantially only composed of the monomers m1, m2, and m3(that is, the total amount of the monomers m1, m2, and m3 issubstantially 100% by weight based on the total amount of the monomermixture). By this adhesive composition, an effect that apressure-sensitive adhesive sheet having good cohesive force andrepelling resistance can be formed can be obtained with a simplecomposition.

As for the technology as disclosed herein, the monomer mixture does notsubstantially contain a carboxyl group-containing monomer. Here, the“carboxyl group-containing monomer” refers to a vinyl monomer (anethylenically unsaturated monomer) having at least one carboxyl groupwithin one molecule thereof (which may be in the form of an anhydride).Specific examples of the carboxyl group-containing monomer includeethylenically unsaturated monocarboxylic acids such as (meth)acrylicacid and crotonic acid; ethylenically unsaturated dicarboxylic acidssuch as maleic acid, itaconic acid and citraconic acid; and anhydridesof ethylenically unsaturated dicarboxylic acids such as anhydrous maleicacid and anhydrous itaconic acid. Further, the expression that themonomer mixture “does not substantially contain” a carboxylgroup-containing monomer means that monomer mixture does not contain anycarboxyl group-containing monomer, or the amount thereof is 0.1% byweight or less based on the total amount of the monomer mixture.

It is preferable that the monomer mixture does not substantially containa carboxyl group-containing monomer, as well as that the monomer mixturedoes not substantially contain an acidic group-containing monomer (asulfonic acid group, a phosphoric acid group, and the like) in additionto the carboxyl group. That is, it is preferable that the monomer doesnot contain an acidic group-containing monomer in addition to thecarboxyl group-containing monomer (which means that the monomer does notcontain a carboxyl group-containing monomer and another acidicgroup-containing monomer), or that the total amount thereof is 0.1% byweight or less based on the total amount of the monomer mixture. Thisadhesive composition can form a pressure-sensitive adhesive layer whichthe corrosion of metal surface is highly protected when it is attachedon the metal surface (that is, the corrosiveness on a metal surface ishighly inhibited).

The monomer mixture can contain other monomers (that is, a monomer otherthan the monomers m1 to m3, which may be hereinafter referred to as a“monomer m4”) as optional components, in addition to the monomers m1 tom3. By using the monomer m4, for example, various characteristics of thepressure-sensitive adhesive, the structure of the acrylic copolymer, orthe like can be more appropriately controlled. As the monomer m4, one ormore selected from various monomers that are copolymerizable with alkyl(meth)acrylate used herein, and do not have a carboxyl group (typicallya carboxyl group and an acidic group other than a carboxyl group) can beemployed. For example, various monomers that have one or moreethylenically unsaturated groups, for example, a (meth)acryloyl group, avinyl group, and the like can also be used.

Examples of the monomer m4 include epoxy group-containing monomers suchas glycidyl (meth)acrylate and allylglycidyl ether; alkoxygroup-containing monomers such as methoxyethyl (meth)acrylate,methoxypropyl (meth)acrylate, (meth)acrylic acid methoxyethyleneglycoland (meth)acrylic acid methoxypolypropyleneglycol; cyanogroup-containing monomers such as acrylonitrile and methacrylonitrile;styrene-based monomers such as styrene and α-methylstyrene; α-olefinssuch as ethylene, propylene, isoprene, butadiene and isobutylene;isocyanate group-containing monomers such as2-methacryloyloxyethylisocyanate; vinyl ester-based monomers such asvinyl acetate and vinyl propionate; vinyl ether-based monomers such asvinyl ether; heterocyclic group-containing (meth)acrylic esters such astetrahydrofurfuryl (meth)acrylate; halogen atom-containing monomers suchas fluorine(meth)acrylate; alkoxysilyl group-containing monomers such as3-methacryloxypropyltrimethoxysilane and vinyltrimethoxysilane; siloxanebond-containing monomers such as silicone (meth)acrylate; alkyl(meth)acrylate in which R² in the formula (I) is an alkyl group having21 or more carbon atoms; alicyclic hydrocarbon group-containing(meth)acrylates such as cyclopentyl (meth)acrylate, cyclohexyl(meth)acrylate, bornyl (meth)acrylate and isobornyl (meth)acrylate; andaromatic hydrocarbon group-containing (meth)acrylates such as phenyl(meth)acrylate, benzyl (meth)acrylate, phenoxyethyl (meth)acrylate andphenoxydiethyleneglycol (meth)acrylate.

Other examples of the monomer m4 include nitrogen atom-containingmonomers other than the monomer m2 and the monomer m3. Examples thereofinclude N-acryloyl group-containing cyclic (meth)acrylamides such asN-(meth)acryloylmorpholine and N-acryloylpyrrolidine; aminogroup-containing monomers such as aminoethyl (meth)acrylate,N,N-dimethylaminoethyl (meth)acrylate and N,N-dimethylaminopropyl(meth)acrylate; maleimide backbone-containing monomers such asN-cyclohexylmaleimide and N-phenylmaleimide; and itaconimide-basedmonomers such as N-methylitaconimide, N-ethylitaconimide,N-butylitaconimide, N-2-ethylhexylitaconimide, N-laurylitaconimide andN-cyclohexylitaconimide.

Furthermore, as the monomer m4, for example, polyfunctional monomerssuch as ethyleneglycol di(meth)acrylate, diethyleneglycoldi(meth)acrylate, triethyleneglycol di(meth)acrylate,tetraethyleneglycol di(meth)acrylate, polyethyleneglycoldi(meth)acrylate, polypropyleneglycol di(meth)acrylate, neopentylglycoldi(meth)acrylate, hexanediol di(meth)acrylate, pentaerythritoldi(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritoltri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, epoxyacrylate,polyesteracrylate, urethaneacrylate, divinylbenzene, butyldi(meth)acrylate and hexyl di(meth)acrylate may be used.

Other examples of the monomer m4 include hydroxyl group-containingmonomers, such as hydroxyalkyl (meth)acrylates such as2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate,3-hydroxypropyl(meth)acrylate, 2-hydroxybutyl(meth)acrylate,4-hydroxybutyl(meth)acrylate, 6-hydroxyhexyl(meth)acrylate,8-hydroxyoctyl(meth)acrylate, 10-hydroxydecyl(meth)acrylate,12-hydroxylauryl(meth)acrylate and [4-(hydroxymethyl)cyclohexyl]methylacrylate; N-hydroxyalkyl(meth)acrylamides in which R⁴ in the formula(II) is a hydroxyalkyl group having 1 or 5 or more carbon atoms; andalkenyl alcohols such as vinyl alcohol and allyl alcohol.

Moreover, in case where a hydroxyl group-containing monomer is used asthe monomer m4, it is preferable to use a lower proportion of thehydroxyl group-containing monomer m4 than that of the monomer m2, fromthe viewpoint that the effect from the use of the monomer m2 can be moreappropriately exhibited. In other words, it is preferable that themonomer m2 constitutes more than 50% by weight of the proportion(typically 60% by weight or more, more preferably 75% by weight or more,for example 90% by weight or more of the proportion) based on the totalamount of the hydroxyl group-containing monomers contained in themonomer mixture. Alternatively, the hydroxyl group-containing monomercontained in the monomer mixture may be substantially only the monomerm2.

The amount of the monomer m4 (in case where two or more kinds arecontained, it is the total amount thereof) is appropriately about 30% byweight or less based on the total amount of the monomer mixture. Whenthe amount of the monomer m4 is too high, a pressure-sensitive adhesivesheet formed by using the composition may have deteriorated balanceamong the pressure-sensitive adhesive performances. The amount of themonomer m4 is preferably about 10% by weight or less, more preferablyabout 5% by weight or less (for example, about 2% by weight or less),based on the total amount of the monomer mixture. Alternatively, themonomer mixture may not substantially contain a monomer m4 (that is, themonomer mixture that contain substantially only the monomers m1 to m3).

The monomer mixture for the technology as disclosed herein preferablycontains the monomers at the proportions, respectively, at which Tg ofthe acrylic copolymer formed by polymerization of approximately entireportion of the monomer mixture is about −10° C. or lower (typicallyabout −10° C. to −70° C.), and more preferably contains the monomers atthe proportions, respectively, at which Tg of the acrylic copolymerobtained by polymerization of the monomer mixture is about −20° C. orlower (typically about −20° C. to −70° C.). The composition of themonomer mixture may be adjusted such that the Tg is in theabove-described range. Here, the Tg of acrylic copolymer refers to avalue as determined from a Fox equation, on the basis of the Tg of thehomopolymer of the monomers constituting the monomer mixture, and theweight fraction of the monomers (copolymerization composition). Thevalue of the Tg of the homopolymer can be obtained from various knownmaterials (“Handbook of Adhesion Technologies” of NIKKAN KOGYO SHIMBUN,LTD., “Polymer Handbook” in Wiley-Interscience, and the like).

A suitable embodiment of the pressure-sensitive adhesive composition asdisclosed herein is a composition containing, as main ingredients, themonomer mixture or a partially polymerized product thereof (in otherwords, an acrylic copolymer material including the unpolymerizedmonomers). For example, it may be a pressure-sensitive adhesivecomposition contains the unpolymerized polymerizable functional groupremaining in a significant amount (for example, about 50 to 100 mol % ofthe monomer mixture). This type of the pressure-sensitive adhesivecomposition is typically configured such that it may form apressure-sensitive adhesive layer (including, as a base polymer, anacrylic copolymer obtained by polymerization of the monomer mixture), byapplying the pressure-sensitive adhesive composition on a substrate tobe described later, a release liner, or the like (which may be referredto as a “substrate”), and reacting a polymerizable functional group inthe applied composition (polymerization) to cure the composition. Thecuring of the pressure-sensitive adhesive composition is preferablycarried out while avoiding the contact with oxygen (for example, underan inert gas atmosphere such as a nitrogen gas).

The polymerization method (a method for curing the composition) in acase where the pressure-sensitive adhesive layer is formed with thepressure-sensitive adhesive composition is not particularly limited, butany method selected from various conventionally known polymerizationmethods can be appropriately employed. For example, any method selectedfrom a polymerization method carried out by the irradiation of activeenergy rays such as light and a radioactive ray onto the composition(which may be hereinafter referred to a high energy ray); apolymerization method carried out using a thermal polymerizationinitiator (thermal polymerization methods such as a solutionpolymerization method, an emulsion polymerization method and a blockpolymerization method); and the like can be employed. As an example ofpreferable curing methods, a method irradiating an active energy ray(for example, light such as ultraviolet ray) can be mentioned. Thepressure-sensitive adhesive composition as disclosed herein can bepreferably carried out in the form of a composition configured to becured by irradiation of such an active energy ray. This active energyray-curable (typically photocurable) adhesive composition isadvantageous in that it is capable of easily forming a thick adhesivelayer. Further, such a pressure-sensitive adhesive composition issuitable to be provided in the form of a liquid composition containingsubstantially no organic solvent (of a non-solvent type). This ispreferable from the viewpoint of environment friendliness, and the like.

This photocurable adhesive composition is typically prepared by using aphotopolymerization initiator. For example, it can be in the forms of apressure-sensitive adhesive composition obtained by mixing the monomermixture and a photopolymerization initiator, and if necessary, blendingwith other components; a pressure-sensitive adhesive compositionobtained by irradiating a mixture including the monomer mixture and aphotopolymerization initiator with light to form a partially polymerizedproduct of the monomer mixture, and if necessary, blending the partiallypolymerized product with other components; a pressure-sensitive adhesivecomposition obtained by further adding (post-adding) aphotopolymerization initiator to the partially polymerized product; apressure-sensitive adhesive composition obtained by forming a partiallypolymerized product of the monomer mixture in a method other thanphotopolymerization, and adding a photopolymerization initiator, and ifnecessary, other components to the partially polymerized product; andthe like.

The photopolymerization initiator is not particularly limited, but itcan be appropriately selected from various conventionally knownmaterials in the field of a photocurable adhesive composition. Examplesof the photopolymerization initiator include a ketal-basedphotopolymerization initiator, an acetophenone-based photopolymerizationinitiator, a benzoin ether-based photopolymerization initiator, anacylphosphine oxide-based photopolymerization initiator, anα-ketol-based photopolymerization initiator, an aromaticsulfonylchloride-based photopolymerization initiator, a photoactiveoxime-based photopolymerization initiator, a benzoin-basedphotopolymerization initiator, a benzyl-based photopolymerizationinitiator, a benzophenone-based photopolymerization initiator and athioxanthone-based photopolymerization initiator. Thesephotopolymerization initiators may be used alone or in combinationthereof.

Specific examples of the ketal-based photopolymerization initiatorinclude 2,2-dimethoxy-1,2-diphenylethan-1-one [for example, trade name“Irgacure 651” (manufactured by Ciba Japan)]. Specific examples of theacetophenone-based photopolymerization initiator include1-hydroxycyclohexylphenylketone [for example, trade name “Irgacure 184”(manufactured by Ciba Japan)], 2,2-diethoxyacetophenone,2,2-dimethoxy-2-phenylacetophenone, 4-phenoxydichloroacetophenone and4-(t-butyl)dichloroacetophenone. Specific examples of thebenzoinether-based photopolymerization initiator includebenzoinmethylether, benzoinethylether, benzoinpropylether,benzoinisopropylether and benzoinisobutylether. As theacylphosphineoxide-based photopolymerization initiator, a trade name“Lucirin TPO” (manufactured by BASF), or the like can be used. Specificexamples of the α-ketol-based photopolymerization initiator include2-methyl-2-hydroxypropiophenone and1-[4-(2-hydroxyethyl)phenyl]-2-methylpropane-1-one. Specific examples ofthe aromatic sulfonyl chloride-based photopolymerization initiatorinclude 2-naphthalenesulfonylchloride. Specific examples of thephotoactive oxime-based photopolymerization initiator include1-phenyl-1,1-propanedione-2-(o-ethoxycarbonyl)-oxime. Specific examplesof the benzoin-based photopolymerization initiator include benzoin.Specific examples of the benzyl-based photopolymerization initiatorinclude benzil. Specific examples of the benzophenone-basedphotopolymerization initiator include benzophenone, benzoyl benzoicacid, 3,3′-dimethyl-4-methoxybenzophenone, polyvinylbenzophenone andα-hydroxycyclohexylphenylketone. Specific examples of thethioxanthone-based photopolymerization initiator include thioxanthone,2-chlorothioxanthone, 2-methylthioxanthone, 2,4-dimethylthioxanthone,isopropylthioxanthone, 2,4-diisopropylthioxanthone anddodecylthioxanthone.

The amount of the photopolymerization initiator to be used is notparticularly limited. For example, the initiator may be used in anamount of about 0.001 to 5 parts by weight (preferably about 0.01 to 2parts by weight, more preferably about 0.01 to 1 part by weight) basedon 100 parts by weight of the total monomer mixture. Further, as usedherein, the expression of the amount of the photopolymerizationinitiator to be used refers to the total amount of thephotopolymerization initiator used in a process for preparing thepressure-sensitive adhesive composition. Accordingly, for apressure-sensitive adhesive composition that is obtained by post-addinga photopolymerization initiator to a partially polymerized productobtained by irradiating a mixture of the monomer mixture and aphotopolymerization initiator with light, it refers to the total amountof a fraction used for the partial polymerization and a fractionpost-added.

The polymerization method for forming a partially polymerized product ofthe monomer mixture is not particularly limited, but any method selectedfrom various conventionally known polymerization methods can beappropriately employed in the same manner as for the polymerizationmethod (curing method) in a case where a pressure-sensitive adhesivelayer is formed with the pressure-sensitive adhesive composition. Forexample, photopolymerization method in which a mixture obtained bymixing the monomer mixture and a photopolymerization initiator, andother components used as needed (a mixture substantially not containinga liquid medium such as an organic solvent, water, and the like (of anon-solvent type) is preferred) is irradiated with light can bepreferably employed. The formation of the partially polymerized productis preferably carried out while avoiding the contact with oxygen (forexample, under an inert gas atmosphere such as a nitrogen gas, and thelike). Further, the polymerization method in a case of forming apartially polymerized product from the monomer mixture, and thepolymerization method in a case of forming a pressure-sensitive adhesivelayer from the pressure-sensitive adhesive composition having, as mainingredients, the partially polymerized product (that is, apolymerization method in a case of preparing a pressure-sensitiveadhesive sheet using the composition) may be the same as or differentfrom each other.

In a case where the pressure-sensitive adhesive composition as disclosedherein is a composition having, as main ingredients, a partiallypolymerized product (typically a photocurable adhesive composition), theconversion of the monomer mixture in the partially polymerized product(preferably a composition in a syrup state in which a polymer formed bypolymerization of a part of the monomer mixture and the unpolymerizedmonomers coexist, and this partially polymerized product having such astate and property may be referred to as the “monomer syrup”) can be,for example, about 2 to 40% by weight. Usually, the conversion ispreferably about 5 to 20%. When the conversion is too high, it is easyto deteriorate the handlability of the pressure-sensitive adhesivecomposition. For example, there is a tendency that it is hard to obtaina composition in the form substantially not containing a liquid medium(of a non-solvent type) that exhibits good paintability at a normaltemperature. On the other hand, when the conversion is too low, thecharacteristics of the pressure-sensitive adhesive obtained by curing(typically photocuring) the pressure-sensitive adhesive composition arelikely to be unstable, and further, the viscosity of the composition maybe too low to be applied.

Further, the conversion of the partially polymerized product isdetermined in the following method. That is, about 0.5 g of a sample istaken from the partially polymerized product, and weighed (weight:W_(p1)). Then, the sample is heated to 130° C. for 2 hours to volatilethe unpolymerized monomers, and the sample remaining after heating isweighed (weight: W_(p2)). Further, the conversion can be determined byapplying each of the obtained values to the following equation:Conversion[%]=(W _(p2) /W _(p1))×100.

The pressure-sensitive adhesive composition as disclosed herein can be,for example, a composition in the form in which an acrylic copolymerobtained by polymerization of approximately all of the monomer mixtureis dissolved or dispersed in a suitable liquid medium (which may be anorganic solvent, water, or a mixture thereof) (of a solvent-type, anaqueous solution type, an emulsion type, and the like). These forms ofthe pressure-sensitive adhesive compositions are typically configuredsuch that it may form a pressure-sensitive adhesive layer having, as abase polymer, the acrylic copolymer, by applying the pressure-sensitiveadhesive composition on a suitable substrate, and the like, and removingthe solvent from the coated product (that is, drying the composition).At this time, a suitable crosslinking treatment, and the like may becarried out, if necessary.

For the pressure-sensitive adhesive composition containing, as mainingredients, an acrylic copolymer obtained by polymerization ofapproximately all of the monomer mixture (which may be an acryliccopolymer material containing substantially no unpolymerized monomers),a method for polymerizing the monomer mixture is not particularlylimited, but various conventionally known polymerization methods can beappropriately employed in the same manner as in a case where the monomermixture are partially polymerized. For example, if the acrylic copolymeris formed, for example, by a solution polymerization method, theembodiment of the solution polymerization is not particularly limited,and can be carried out by appropriately employing, for example, variousknown monomer supplying methods, polymerization conditions (apolymerization temperature, a polymerization time, a polymerizationpressure, and the like), the materials to be used (a polymerizationinitiator, surfactant, and the like), by the same embodiment as theconventionally known common solution polymerization. As the monomersupplying method, any method such as a batch feed method for supplying awhole amount of the monomer mixture at once to a reaction vessel, acontinuous supplying (dropping) method, a divided supplying (dropping)method, or the like can be employed. In a preferred embodiment, anembodiment in which a solution obtained by dissolving a whole amount ofthe monomer mixture and an initiator in a solvent is prepared in areaction vessel, and then the monomer mixture are polymerized batchwise(batch polymerization) is exemplified. This batch polymerization ispreferred, since it is easy to carry out the polymerization operationand the process control. In another preferred embodiment, an embodimentin which an initiator (typically a solution obtained by dissolving aninitiator in a solvent) is prepared in a reaction vessel, and thenmonomer mixture are polymerized while dropping the solution dissolved inthe solvent onto the reaction vessel (dropping polymerization orcontinuous polymerization) is exemplified. A part (some of kinds and/orpartial proportion) of the monomer mixture is typically put into thereaction vessel with the solvent, and then the residual mixture may bedropped onto the reaction vessel.

Examples of the thermal polymerization initiator include azo-basedcompounds (azo-based initiators) such as 2,2′-azobisisobutyronitrile,2,2′-azobis-2-methylbutyronitrile, dimethyl2,2′-azobis(2-methylpropionate), 4,4′-azobis-4-cyanovaleric acid,azobisisovaleronitrile, 2,2′-azobis(2-amidinopropane)dihydrochloride,2,2′-azobis[2-(5-methyl-2-imidazolin-2-yl)propane]dihydrochloride,2,2′-azobis(2-methylpropionamidine)disulfate,2,2′-azobis(N,N′-dimethyleneisobutylamidine)dihydrochloride and2,2′-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]hydrate;persulfates such as potassium persulfate and ammonium persulfate;peroxides (peroxide-based initiators) such as dibenzoyl peroxide,tert-butyl permaleate, t-butyl hydroxyperoxide and hydrogen peroxide;substituted ethane-based initiators such as phenyl-substituted ethane;redox system initiators such as a combination of a persulfate and sodiumhydrogen sulfite, and a combination of a peroxide and sodium ascorbate.In a case where the monomer mixture is polymerized by the thermalpolymerization method, a polymerization temperature at, for example,around 20 to 100° C. (typically 40 to 80° C.) can be appropriatelyemployed.

In a preferred embodiment of the pressure-sensitive adhesive compositionas disclosed herein, the composition contains a crosslinking agent. Byusing the crosslinking agent, a pressure-sensitive adhesive layer formedfrom the composition can be provided with suitable cohesive force andadhesive force, and the repelling resistance of the pressure-sensitiveadhesive layer can also be improved. Any crosslinking agent can beappropriately selected from various conventionally known materials inthe field of a pressure-sensitive adhesive. For example, anisocyanate-based compound (an isocyanate-based crosslinking agent), anepoxy-based crosslinking agent, an aziridine-based crosslinking agent, amelamine-based crosslinking agent, a metal chelate-based crosslinkingagent, a metal salt-based crosslinking agent, a peroxide-basedcrosslinking agent, an oxazoline-based crosslinking agent, an urea-basedcrosslinking agent, an amino-based crosslinking agent, acarbodiimide-based crosslinking agent, a coupling agent-basedcrosslinking agent (for example, a silane coupling agent), or the likecan be used. These crosslinking agents may be used alone or incombination thereof. This crosslinking agent can be preferably blendedinto the composition in the form in which an acrylic copolymer formed bypolymerization of approximately all of the monomer mixture is dissolvedor dispersed in a suitable liquid medium as in the solvent-type adhesivecomposition. Among these, the isocyanate-based crosslinking agent ispreferably used. In a preferred embodiment, as the crosslinking agent,only one or more (typically one) isocyanate-based crosslinking agent isused. Alternatively, within a range not clearly interfering with theeffect of the present invention, the crosslinking agents other than theisocyanate-based crosslinking agent can be used in combination with theisocyanate-based crosslinking agent.

Examples of the isocyanate-based compound include aliphaticpolyisocyanates such as 1,6-hexamethylene diisocyanate,1,4-tetramethylene diisocyanate, 2-methyl-1,5-pentane diisocyanate,3-methyl-1,5-pentane diisocyanate, lysine diisocyanate, and the like;alicyclic polyisocyanates such as isophorone diisocyanate, cyclohexyldiisocyanate, hydrogenated tolylene diisocyanate, hydrogenated xylenediisocyanate, hydrogenated diphenylmethane diisocyanate, hydrogenatedtetramethylxylene diisocyanate, and the like; aromatic polyisocyanatessuch as 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate,4,4′-diphenylmethane diisocyanate, 2,4′-diphenylmethane diisocyanate,4,4′-diphenylether diisocyanate, 2-nitrodiphenyl-4,4′-diisocyanate,2,2′-diphenylpropane-4,4′-diisocyanate,3,3′-dimethyldiphenylmethane-4,4′-diisocyanate, 4,4′-diphenylpropanediisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate,naphthylene-1,4-diisocyanate, naphthylene-1,5-diisocyanate,3,3′-dimethoxydiphenyl-4,4′-diisocyanate, and the like; aromatic,aliphatic polyisocyanates such as xylene-1,4-diisocyanate,xylene-1,3-diisocyanate, and the like; etc.

Further, as the isocyanate-based crosslinking agent, a dimer or trimer,a reaction product, or a polymerized product of the above-exemplifiedisocyanate-based compound (for example, a dimer or trimer ofdiphenylmethane diisocyanate, a reaction product of trimethylolpropaneand tolylene diisocyanate, a reaction product of trimethylolpropane andhexamethylene diisocyanate, polymethylenepolyphenylisocyanate,polyetherpolyisocyanate, polyesterpolyisocyanate), or the like can beused. For example, a reaction product of trimethylolpropane and tolylenediisocyanate can be preferably used.

The amount of the isocyanate-based crosslinking agent to be used can be,for example, about 0.01 to 20 parts by weight (preferably about 0.01 to15 parts by weight), based on 100 parts by weight of the monomer mixture(in a case of the polymerization conversion of the monomer mixture inthe pressure-sensitive adhesive composition is about 100%, it usuallycorresponds to 100 parts by weight of an acrylic copolymer). When theamount of the crosslinking agent to be used is too low, it is hard toexhibit a sufficient effect (an effect of improving thepressure-sensitive adhesive performance), and when the amount thereof tobe used is too high, it is easy to spoil the balance among the adhesioncharacteristics. Usually, it is suitable to use about 0.01 to 1 parts byweight (preferably about 0.02 to 1 parts by weight, for example, about0.05 to 0.5 parts by weight) of the isocyanate-based crosslinking agent,based on 100 parts by weight of the monomer mixture.

In a case where the pressure-sensitive adhesive composition as disclosedherein is a composition contains, as the main ingredients, the monomermixture or a partially polymerized product thereof (typically, aphotocurable adhesive composition), polyfunctional (meth) acrylate (thatis, a monomer containing two or more (meth)acryloyl groups within onemolecule thereof) can be preferably used as the crosslinking agent. Forexample, one or more selected from the polyfunctional (meth)acrylatesexemplified in the description on the monomer m4 can be used. Suitableexamples thereof include 1,6-hexanediol di(meth)acrylate,trimethylolpropane tri(meth)acrylate, pentaerythritoltetra(meth)acrylate and 1,2-ethylenglycol di(meth)acrylate. From theviewpoint of the polymerization reactivity (crosslinking reactivity),and the like, polyfunctional acrylate is preferably used. For thepressure-sensitive adhesive composition having a partially polymerizedproduct of the monomer mixture as the main ingredients, it is preferableto partially polymerizing the monomer mixture, and then blendingpolyfunctional monomers therewith (that is, post-addition). In apreferred embodiment, as the crosslinking agent, only one or more(typically one) polyfunctional (meth)acrylate is used. Alternatively,within a range not clearly interfering with the effect of the presentinvention, a crosslinking agent other than the polyfunctional(meth)acrylate (for example, an isocyanate-based crosslinking agent) canbe used in combination with the polyfunctional (meth)acrylate.

The amount of the polyfunctional (meth) acrylate to be used for blendingas a crosslinking agent is, for example, about 0.001 to 5 parts byweight, and usually suitably about 0.005 to 1 part by weight (forexample, about 0.005 to 0.1 part by weight), based on 100 parts byweight of the monomer mixture. When the amount of the polyfunctional(meth)acrylate to be used is too low, a sufficient crosslinking effectis not exhibited, and there is a tendency that the cohesive force or therepelling resistance is reduced. On the other hand, when the amount ofthe polyfunctional (meth)acrylate to be used is too high, the elasticityof the pressure-sensitive adhesive formed after curing increases toohigh, and the adhesive force or the tackiness may be easily reduced.

In the pressure-sensitive adhesive layer provided on thepressure-sensitive adhesive sheet as disclosed herein, thepressure-sensitive adhesive layer preferably includes apressure-sensitive adhesive having a gel fraction of, for example, about25 to 75%. In order to form a pressure-sensitive adhesive having the gelfraction (a pressure-sensitive adhesive after crosslinking in thecomposition including a crosslinking agent), the conditions such as themonomer composition (for example, the amount of the monomer m2 to beused), the polymerization degree of the monomer mixture in thepressure-sensitive adhesive composition, the molecular weight of theacrylic copolymer, the condition for forming the pressure-sensitiveadhesive layer (drying condition, light irradiation condition, and thelike), the kind and the amount of the crosslinking agent to be used, andthe like may be appropriately set. When the gel fraction of thepressure-sensitive adhesive is too low, there is a tendency ofinsufficient cohesive force or repelling resistance. On the other hand,when the gel fraction thereof is too high, the pressure-sensitiveadhesive force or the tackiness may be easily reduced. By thepressure-sensitive adhesive having a gel fraction in a range of about 25to 75% (for example, about 30 to 60%), better adhesive performances canbe realized.

As used herein, “the gel fraction of the pressure-sensitive adhesive”refers to a value as measured in the following method. The gel fractioncan be understood as a weight ratio of the ethyl acetate-insolublecomponents in the pressure-sensitive adhesive.

Method for Measuring Gel Fraction

A pressure-sensitive adhesive sample (weight: W_(b1)) is enclosed with aporous polytetrafluoroethylene film (weight: W_(b2)) having an averagepore diameter of 0.2 μm in a bag shape, and the opening part thereof istied with a kite string (weight W_(b3)). This bag is immersed in 50 mLof ethyl acetate, and left to stand at room temperature (typically 23°C.) for 7 days. Then, the bag is taken out, and ethyl acetate remainingon the outer surface of the bag is wiped out. The bag is dried at 130°C. for 2 hours, and the weight of the bag (W_(b4)) is measured. The gelfraction of the pressure-sensitive adhesive is determined by applyingeach value to the following equation:Gel Fraction[%]=[(W _(b4) −W _(b2) −W _(b3))/W _(b1)]×100

Further, it is desirable to use a trade name “NITOFLON (registeredtrademark) NTF1122” (average pore diameter: 0.2 μm, porosity: 75%, andthickness: 85 μm), or an equivalent one thereof, available from NittoDenko Corporation, as the porous polytetrafluoroethylene (PTFE) film.

The pressure-sensitive adhesive composition as disclosed herein cancontain, as an optional component, various additives that are common inthe field of the pressure-sensitive adhesive compositions. Examples ofthis optional component include a tackifier (a rosin-based resin, apetroleum-based resin, a terpene-based resin, a phenol-based resin, aketone-based resin, and the like), a plasticizer, a softener, a filler,a colorant (a pigment, a dye, and the like), an antioxidant, a labelingagent, a stabilizer and a preservative. These additives can be used by ausual method, employing ones conventionally known, and do notparticularly characterize the present invention, and hence the detaileddescription thereon is omitted here.

Furthermore, the pressure-sensitive adhesive composition as disclosedherein can be appropriately blended with a polymer, apart from thecopolymer formed by polymerization of the monomer mixture for thepurpose of adjusting the viscosity (typically of thickening). The use ofthe polymer for adjusting the viscosity is particularly effective forthe pressure-sensitive adhesive composition (typically, the photocurableadhesive composition), having, as main ingredients, the monomer mixtureor a partially polymerized product thereof. Examples of the polymer foradjusting the viscosity include a styrene butadiene rubber (SER),isoprene rubber (IR), a styrene butadiene-styrene block copolymer (SBS),an ethylene-acetic acid vinyl copolymer, an acryl rubber, polyurethane,polyester, and the like. In addition, an acrylic polymer obtained bycopolymerizing alkyl (meth)acrylate with a functional monomer (forexample, one or more selected from the acrylic monomers havingfunctional groups such as acrylamide, acrylonitrile andacryloylmorpholine, acrylic acid) may be used as the polymer foradjusting the viscosity. It is preferable to employ a polymer foradjusting the viscosity, which does not substantially contain a carboxylgroup (more preferably a carboxyl group and an acidic group other thanthe carboxyl group).

These polymers for adjusting the viscosity may be used alone or incombination thereof, but they are preferably used in an amount in arange of about 40% by weight or less (typically about 5 to 40% byweight) based on the entire pressure-sensitive adhesive formed with thepressure-sensitive adhesive composition. That is to say, it ispreferable that the proportion of the polymer for adjusting theviscosity is about 40% by weight or less (more preferably about 20% byweight or less) of the pressure-sensitive adhesive forming componentscontained in the composition.

The pressure-sensitive adhesive composition as disclosed herein ispreferably configured such that about 50% by weight or more (morepreferably about 70% by weight or more, for example 90% by weight ormore) of the acrylic copolymer obtained by polymerization of the monomermixture is contained in a pressure-sensitive adhesive formed with thecomposition. This adhesive composition may form a pressure-sensitiveadhesive with better adhesive performance.

The pressure-sensitive adhesive sheet according to the present inventionincludes a pressure-sensitive adhesive layer formed with any one of thepressure-sensitive adhesive compositions as disclosed herein. It may bea pressure-sensitive adhesive sheet attached on the substrate, in whichsuch a pressure-sensitive adhesive layer is provided as affixed onesurface or both surfaces of a sheet-like substrate (support) (without anintention for separating the pressure-sensitive adhesive layer from thesubstrate), or a pressure-sensitive adhesive sheet without a substrate,in which the pressure-sensitive adhesive layer is provided on a supporthaving releasability, for example, a release liner (a release paper, aresin sheet having its surface subjected to release treatment, and thelike). As for the concept of the pressure-sensitive adhesive sheet asreferred to herein, those referred to a pressure-sensitive adhesivetape, a pressure-sensitive adhesive label, a pressure-sensitive adhesivefilm, and the like are encompassed. In addition, the pressure-sensitiveadhesive layer is not limited to ones that are continuously formed, butit may be adhesive layers formed for example, in the regular patternssuch as a point form, a stripe form, and the like, or in the randompatterns.

The pressure-sensitive adhesive sheet as disclosed herein may be, forexample, one having a cross-section structure as schematically shown inFIGS. 1 to 6. Among these, FIG. 1 and FIG. 2 are examples ofconfigurations of a double-sided adhesive type of a pressure-sensitiveadhesive sheet attached on a substrate. The pressure-sensitive adhesivesheet 11 as shown in FIG. 1 includes the pressure-sensitive adhesivelayers 2 on the both sides of the substrate 1, and thepressure-sensitive adhesive layers 2 are each configured such that theyare protected with a release liner 3 having at least the side of thepressure-sensitive adhesive layer with a release surface. Thepressure-sensitive adhesive sheet 12 as shown in FIG. 2 is configuredsuch that the both sides of the substrate 1 are provided with thepressure-sensitive adhesive layers 2, and at least one of these adhesivelayers is protected with a release liner 3 having both sides of thepressure-sensitive adhesive layer with a release surface. This kind ofthe pressure-sensitive adhesive sheet 12 can be configured such that thepressure-sensitive adhesive layer on the other side is contacted withthe back side of the release liner 3 when the pressure-sensitiveadhesive sheet 12 is wound, and the pressure-sensitive adhesive layer onthe other side is also protected with the release liner 3.

FIG. 3 and FIG. 4 are examples of the configurations of thepressure-sensitive adhesive sheet without a substrate. Thepressure-sensitive adhesive sheet 13 as shown in FIG. 3 is configuredsuch that the both sides of the pressure-sensitive adhesive layer 2without a substrate are protected with a release liner 3 having at leastthe side of the pressure-sensitive adhesive layer with a releasesurface. The pressure-sensitive adhesive sheet 14 as shown in FIG. 4 canbe configured such that one side of the pressure-sensitive adhesivelayer 2 without a substrate is protected with a release liner 3 havingthe both sides with a release surface, and when this sheet is wound, theother side of the pressure-sensitive adhesive layer 2 is contacted withthe release liner 3, and the other side is also protected with therelease liner 3.

FIG. 5 and FIG. 6 are examples of the configurations of a one-sidedadhesive type of a pressure-sensitive adhesive sheet attached on asubstrate. The pressure-sensitive adhesive sheet 15 as shown in FIG. 5is configured such that one side of the substrate 1 is provided with thepressure-sensitive adhesive layer 2, and the surface (adhered surface)of the pressure-sensitive adhesive layer 2 is protected with the releaseliner 3 having at least the side of the pressure-sensitive adhesivelayer with a release surface. The pressure-sensitive adhesive sheet 16as shown in FIG. 6 is configured such that one side of the substrate 1is provided with the pressure-sensitive adhesive layer 2. It can beconfigured such that the other side of the substrate 1 has a releasesurface, and if the pressure-sensitive adhesive sheet 16 is wound, thepressure-sensitive adhesive layer 2 is contacted with the other side,and the surface (adhered surface) of the pressure-sensitive adhesivelayer is protected on the other side of the substrate 1.

A substrate constituting the pressure-sensitive adhesive sheet can beappropriately selected, for example, from plastic films such as apolypropylene film, an ethylene-propylene copolymer film, a polyesterfilm and a polyvinyl chloride film; foam substrates such as apolyurethane foam and a polyethylene foam; papers such as a craft paper,a crepe paper and a Japanese paper; cloth such as cotton and stablefibers; non-woven fabrics such as a polyester non-woven fabric and avinylon non-woven fabric; metal foils such as an aluminum foil and acopper foil; and the like according to the applications of thepressure-sensitive adhesive sheet, and used. As the plastic film, bothof a non-stretched film and a stretched (monoaxially stretched orbiaxially stretched) film can be used. Further, the side provided with apressure-sensitive adhesive layer in the substrate may be surfacetreated with a primer application, corona discharge treatment, and thelike. The thickness of the substrate can be appropriately selectedaccording to the purposes, but generally, it is usually from about 10 μmto 500 μm (typically from 10 μm to 200 μm).

The pressure-sensitive adhesive layer can be preferably formed, forexample, by placing (typically applying) any one of thepressure-sensitive adhesive compositions as disclosed herein, that is, apressure-sensitive adhesive compositions containing, as mainingredients, the monomer mixture or a partially polymerized productthereof, on a support (a substrate or a release liner), and carrying outthe curing (polymerization) by irradiating the composition with anactive energy ray (for example, an ultraviolet ray). By the method forforming the pressure-sensitive adhesive layer, a more suitable resultcan be realized, for example, from the viewpoint of the heat resistanceof the pressure-sensitive adhesive layer. In the formation method, anacrylic copolymer obtained by polymerization of the monomer mixture (abase polymer in the pressure-sensitive adhesive layer) is synthesized,and at the same time, a pressure-sensitive adhesive layer is formed.This method can be preferably applied for an active energy ray-curableadhesive composition prepared by blending the monomer mixture (a mixtureof the non-polymerized monomers), or a partially polymerized productobtained by the partial polymerization of the monomer mixture in anappropriate polymerization method (for example, a photopolymerizationmethod) with a photopolymerization initiator, and a crosslinking agentused as needed (polyfunctional (meth)acrylate, and the like). Thisactive energy ray-curable adhesive composition can be a compositionsubstantially not containing a liquid medium (an organic solvent, water,and the like) (of a non-solvent). For the active energy ray-curableadhesive composition in the form including a liquid medium, it ispreferable to dry the composition provided on the support, and thenirradiating an active energy ray thereon.

Moreover, the pressure-sensitive adhesive layer can be also formed byplacing (typically applying) any one of the pressure-sensitive adhesivecompositions as disclosed herein on a support (a substrate or a releaseliner), and drying the composition. The method for forming thepressure-sensitive adhesive layer can be preferably applied for apressure-sensitive adhesive composition in the form in which an acryliccopolymer obtained by polymerization of approximately all of the monomermixture and a crosslinking agent, an additive, and the like used asneeded are dissolved or dispersed in a liquid medium in advance in anappropriate polymerization method (typically a thermal polymerizationmethod). In a case of the pressure-sensitive adhesive compositionincluding the crosslinking agent, appropriate crosslinking may becarried out, if necessary, in addition to the drying as above.

The application of the pressure-sensitive adhesive composition can becarried out using a common coater such as, for example, a gravure rollcoater, a reverse roll coater, a kiss-roll coater, a dip roll coater, abar coater, a knife coater, a spray coater, and the like. From aviewpoint of promoting a crosslinking reaction, improving thepreparation efficiency, and the like, the pressure-sensitive adhesivecomposition can be preferably dried under heating. The temperature fordrying varies depending on the kinds of the support on which thecomposition is applied, but a drying temperature, for example, in arange of about 40 to 150° C. can be employed.

Further, in a case of a pressure-sensitive adhesive sheet attached on asubstrate, the pressure-sensitive adhesive composition may be directlyplaced on a substrate to form a pressure-sensitive adhesive layer, andthe pressure-sensitive adhesive layer formed on a release liner may betransferred on a substrate.

The thickness of the pressure-sensitive adhesive layer is notparticularly limited, but is usually, for example, about 10 μm or more(preferably about 20 μm or more, more preferably about 30 μm or more),whereby good adhesive performances (for example, adhesive strength) canbe realized. Further, usually, it is preferable that the thickness isabout 400 μm or less (typically about 200 μm or less, for example, about100 μm or less).

EXAMPLES

Hereinbelow, several Examples of the present invention are described,but it is not construed the present invention is intended to be limitedto these Examples. Further, in the following description, “parts” and“%” are based on the weight unless otherwise specifically stated.

Example 1

To 100 parts of the monomer mixture including 70 parts of2-ethylhexylacrylate (2EHA, Tg of the homopolymer: −70° C.), 30 parts ofN-vinyl-2-pyrrolidone (NVP, Tg of the homopolymer: 54° C.), and 4 partsof N-hydroxyethylacrylamide (HEAA, Tg of the homopolymer: 98° C.), 0.05part of 2,2-dimethoxy-1,2-diphenylethan-1-one (a trade name “Irgacure651” (manufactured by Ciba Japan)), and 0.05 part of1-hydroxy-cyclohexyl-phenylketone (a trade name “Irgacure 184”(manufactured by Ciba Japan)) as photopolymerization initiators wereblended. The dissolved oxygen was sufficiently removed from this mixtureby stirring under a nitrogen gas atmosphere, and an ultraviolet ray wasirradiated onto the mixture to obtain an acrylic copolymer material(monomer syrup) obtained by the partial polymerization (a conversion(monomer conversion): about 11%) of the monomer mixture. For thismonomer syrup, as the inner crosslinking agent, 0.01 part of1,6-hexanediol diacrylate (HDDA) was added, based on 100 parts of themonomer mixture. As such, the photocurable pressure-sensitive adhesivecomposition according to Example 1 was prepared.

Two sheets of the release liners (thickness: 38 μm) that had beenrelease-treated by a silicone-based releasing agent were prepared on thesurface of a polyethylene terephthalate (PET) film. The above-describedadhesive composition was applied onto the release surface (the surfacethat had been treated with a releasing agent) of the first sheet of therelease liner. At this time, the application amount was adjusted suchthat the finally obtained adhesive layer had a thickness of 50 μm. Then,the applied adhesive composition was stack with the release surface ofthe second release liner. Further, by irradiation of an ultraviolet rayunder the conditions of a light intensity of about 4 mW/cm² and a lightamount of about 720 mJ/cm², the composition was cured to form apressure-sensitive adhesive layer. As such, a pressure-sensitiveadhesive sheet (a release liner-attached adhesive sheet) configured suchthat the both sides of the pressure-sensitive adhesive layer wereprotected with the release liners was prepared.

The gel fraction of the pressure-sensitive adhesive constituting thepressure-sensitive adhesive layer of the pressure-sensitive adhesivesheet according to the present Example was measured in accordance withthe method for measuring a gel fraction in the following manner. Thatis, a porous PTFE film having a size of 100 mm×100 mm (a trade name“NITOFLON (registered trademark) NTF1122”, manufactured by Nitto DenkoCorporation) and a kite string having a length of about 100 mm(thickness: 1.5 mm) were prepared, and the weight thereof were measured.Both the release liners were peeled from a cut size of 20 cm² of therelease liner-attached pressure-sensitive adhesive sheet, and thepressure-sensitive adhesive sample was enclosed with the PTFE film in abag shape, and the opening part thereof was tied with a kite string. Theweight of the bag was measured, and the weight of the PTFE film (W_(b2))and the weight of the kite string (W_(b3)) were subtracted from theweight of the bag to determine the weight of the pressure-sensitiveadhesive sample (W_(b1)). Then, the bag was immersed in 50 mL of ethylacetate, and left to stand at room temperature (typically 23° C.) for 7days. Thereafter, the bag was taken out of ethyl acetate, and ethylacetate remaining on the outer surface of the bag was wiped out. The bagwas dried in a drier at 130° C. for 2 hours, and then the weight of thebag (W_(b4)) was measured. The gel fraction of the pressure-sensitiveadhesive as determined by applying each value to the equation asdescribed earlier was 49.9%.

Examples 2 to 5

A pressure-sensitive adhesive composition was prepared in the samemanner as in Example 1, except that the monomer mixture of thecomposition shown in Table 1 (in the table, “AA” represents acrylicacid; Tg of the homopolymer: 106° C.) was used, and that the amounts ofHDDA to be added were 0.02 part in Example 3 and 0.04 part in Example 5,respectively, and a pressure-sensitive adhesive sheet was prepared basedon 100 parts of the monomer mixture. The gel fractions of thepressure-sensitive adhesive as measured in the same manner as in Example1 were 48% in Example 2, 68.1% in Example 3, 81.9% in Example 4, and 60%in Example 5, respectively.

For the pressure-sensitive adhesive sheet prepared in Examples 1 to 5(which are provided with the pressure-sensitive adhesive layers formedfrom an ultraviolet ray-curable adhesive composition), the followingevaluation tests were carried out.

Corrosiveness

One side of the release liner was peeled from the pressure-sensitiveadhesive sheet according to each of Examples to expose one side of thepressure-sensitive adhesive layer, and a transparent PET film having athickness of 25 μm (which had not been release-treated) was attached andsupported thereon. In addition, the release liner on the other side waspeeled to expose the other side of the pressure-sensitive adhesivelayer, and this was attached and combined with a copper foil having athickness of 80 μm, and then maintained at an atmosphere of 60° C.×95%RH for 250 hours. Thereafter, the surface of the copper foil at aportion attached with the pressure-sensitive adhesive sheet was observedwith naked eyes on the side of the PET film, the presence or absence ofthe corrosion of the surface of the copper foil was confirmed using thecolor change in the surface of the copper foil as an index. As a result,a case where the color change in the surface of the copper foil was notconfirmed was denoted as “absence” of the corrosiveness, whereas a casewhere the color change in the surface of the copper foil was confirmedwas denoted as “presence” of the corrosiveness.

Adhesive Force

One side of the release liner was peeled from the pressure-sensitiveadhesive sheet according to each of Examples to expose one side of thepressure-sensitive adhesive layer, and a PET film having a thickness of50 μm (which had not been release-treated) was attached and supportedthereon. This supported adhesive sheet was cut into a width of 25 mm toprepare a test piece. As an object to be adhered, a clean acrylic plateobtained by reciprocatingly rubbing and washing a clean waste to whichisopropyl alcohol had been immersed 10 times was used. The release lineron the other side was peeled from the test piece, and was pressed ontothe object to be adhered in such a manner that a 5 kg roller was rolledonce. After storing it at 40° C. for 2 days, it was taken out under ameasurement environment of 23° C.×50% RH, left to stand for 30 minutes,and a peeling strength (N/25 mm) was measured using a tension testerunder the condition of a tension rate of 300 mm/min, and a peeling angleof 180°.

Repelling Resistance

The pressure-sensitive adhesive sheet according to each of Examples wascut into a size of a width of 10 mm and a length of 90 mm, and therelease liner was peeled from one side. Then, an aluminum plate havingthe same size (thickness: 0.5 mm) was attached and combined therewith toprepare a test piece. This test piece was bent along a cord of φ50 mm inthe longitudinal direction with the side of an aluminum plate inside(that is, a curvature of R50). Then, the release liner was peeled fromthe other side of the test piece, and pressed such that no floatingoccurred using a laminator on the surface of a polypropylene plate thathad been washed in the same manner as described above. This was left tostand in an environment of 23° C. for 4 hours or 7 hours, and then thefloating height (mm) of an edge of the test piece in the longitudinaldirection over the surface of the polypropylene plate was measured. Themeasurement was carried out on the both ends of the test piece, and thetotal value of the floating heights for the both ends was taken as avalue of repelling resistance.

Holding Power

As an index of the cohesive force, the holding power (static loadcharacteristics) of the pressure-sensitive adhesive sheet according toeach of Examples was evaluated in the following manner. That is, therelease liner on one side was peeled from the pressure-sensitiveadhesive sheet to expose one side of the pressure-sensitive adhesivelayer, and a PET film having a thickness of 50 μm (which had not beenrelease-treated) was attached and supported thereon. This supportedadhesive sheet was cut into a size of a width of 10 mm and a length of50 mm to prepare a test piece. As an object to be adhered, a cleanBakelite plate washed by reciprocatingly rubbing a clean waste in whichtoluene had been immersed 10 times was used. The release liner on theother side was peeled from the test piece, and was pressed onto theobject to be adhered in such a manner that a 2 kg roller was rolledreciprocatingly at a contact area of a width of 10 mm and a length of 20mm. After maintaining it at 40° C. for 30 minutes, the Bakelite platewas taken down under an environment of 40° C., and after giving 500 g ofa load to a free end of the test piece (a protruding portion of theBakelite plate), left to stand under an environment of 40° C. for 2hours. After giving the load and before the lapse of 2 hours thereafter,a case where the test piece was dropped from the object to be adheredwas denoted as “bad” holding power, whereas a case where the test piecewas held in the object to be adhered even after 2 hours had passed wasdenoted as “good” holding power.

The results of the evaluation tests are shown in Table 1. In this table,the composition (the kinds and amount ratios of the monomers to be used)of the monomer mixture used in the preparation of the pressure-sensitiveadhesive composition according to each of Examples are shown togetherwith the Tg as determined from the composition on the basis of the Foxequation.

TABLE 1 Repelling resistance Peeling Monomer mixture (part) Tg (mm)strength Holding 2EHA NVP HEAA AA (° C.) Corrosiveness 4 hr 7 hr (N/25mm) power Example 1 70 30 4 −41 Absence 0 2.2 22.4 Good Example 2 70 264 −43 Absence 0 0.8 23.7 Good Example 3 70 30 −44 Absence 7.9 15.3 23.2Good Example 4 48 48 4 −19 Absence 50 50 1.3 Good Example 5 90 10 −60Presence 0.7 6.2 20 Good

As shown in Table 1, as compared with the pressure-sensitive adhesivesheet having the copolymerization composition including the monomer m1(2EHA) and the monomer m3 (NVP) and not including the monomer m2 in thepresent invention (Example 3), according to the pressure-sensitiveadhesive sheet having the copolymerization composition further includingthe monomer m2 (here, HEAA) in addition to the monomers m1 and m3(Examples 1 and 2), the repelling resistance was remarkably improvedwhile other characteristics were equivalently maintained. As a result,according to the pressure-sensitive adhesive sheets of Examples 1 and 2,equivalent or more levels of the repelling resistance and the peelingstrength were realized, as compared with the pressure-sensitive adhesivesheet of Example 5 obtained by using carboxyl group-containing monomers(here, AA) of general adhesive compositions. Further, thepressure-sensitive adhesive sheet according to Example 4 having a toohigh amount of the monomer m3 was deficient in the balance among theadhesion characteristics. Further, the pressure-sensitive adhesivesheets according to Example 1 to 4 did not have corrosiveness on a metalsurface, and also had sufficient holding power. On the other hand, itwas confirmed that the pressure-sensitive adhesive sheet of Example 5obtained by using AA exhibits better adhesion characteristics than thepressure-sensitive adhesive sheet according to Example 3, but itcorrodes the metal surface.

Examples 6 and 7

Pressure-sensitive adhesive compositions were prepared in the samemanner as in Example 1, except that the monomer mixture of thecomposition shown in Table 2 (in the table, “BA” represents butylacrylate; Tg of the homopolymer; −54° C.) were used, and that the amountof HDDA to be added was 0.03 part for either Example 6 or 7, based 100parts of the monomer mixture, and pressure-sensitive adhesive sheetswere prepared using the compositions. The gel fractions of thepressure-sensitive adhesives were 28.7% in Example 6 and 53% in Example7, respectively.

Examples 8 and 9

Pressure-sensitive adhesive compositions were prepared in the samemanner as in Example 1, except that the monomer mixture of thecomposition shown in Table 2 (in the table, “DEAA” represents NN-diethylacrylamide; Tg of the homopolymer: 81° C.) were used, and that theamounts of HDDA to be added were 0.05 part in Example 8 and 0.06 part inExample 9, respectively, based 100 parts of the monomer mixture, andpressure-sensitive adhesive sheets were prepared using the compositions.The gel fractions of the pressure-sensitive adhesives were 30.2% inExample 8 and 19.7% in Example 9, respectively.

The evaluation tests as described above were carried out on thepressure-sensitive adhesive sheets prepared in Examples 6 to 9 (whichare provided with adhesive layers formed from the ultraviolet-curabletype adhesive compositions). The results are shown in Table 2.

TABLE 2 Repelling resistance Peeling Monomer mixture (part) Tg (mm)strength Holding BA 2EHA NVP DEAA HEAA (° C.) Corrosiveness 4 hr 7 hr(N/25 mm) power Example 6 78 13 9 −45 Absence 1.1 1.4 16.6 Good Example7 78 22 −46 Absence 3.0 9.3 18.0 Good Example 8 70 26 4 −40 Absence 0.10.3 21.3 Good Example 9 70 30 −40 Absence 7.9 16.9 19.3 Bad

As seen from the comparison between Examples 6 and 7, as compared withthe pressure-sensitive adhesive sheet having the copolymerizationcomposition including the monomer m1 (BA) and the monomer m3 (NVP), andnot including the monomer m2 in the present invention (Example 7),according to the pressure-sensitive adhesive sheet having thecopolymerization composition replacing a part of NVP with the monomer m2(here, HEAA) (Example 6), the repelling resistance was clearly improvedwhile other characteristics were equivalently maintained. As seen fromthe comparison between Examples 8 and 9, as compared with thepressure-sensitive adhesive sheet having the copolymerizationcomposition including the monomer m1 (2EHA) and the monomer m3 (DEAA),and not including the monomer m2 in the present invention (Example 9),according to the pressure-sensitive adhesive sheet having thecopolymerization composition replacing a part of DEAA with HEAA (Example8), the repelling resistance and the holding power were clearly improvedwhile the peeling strength was equivalently maintained. Further, all ofthe pressure-sensitive adhesive sheets of Examples 6 to 9 did not havecorrosiveness on a metal surface.

Example 10

To a reaction vessel equipped with a cooling tube, a nitrogen gas inlettube, a thermometer, and a stirrer were put 0.2 part of2,2′-azobisisobutylonitrile (AIBN) as a polymerization initiator, 70parts of 2EHA, 26 parts of NVP, and 4 parts of HEAA as the monomercomponents, and ethyl acetate as a solvent, followed by stirring at roomtemperature under a nitrogen gas atmosphere for 1 hour. Thereafter, thecontents (a solution containing the entire amount of the monomermixture) in the reaction vessel were heated to 60° C., and polymerizedin a nitrogen gas stream for 5.5 hours. By this batch feed type ofsolution polymerization, a solution of the acrylic polymer was obtained.

To the obtained acrylic polymer solution was added a reaction product oftrimethylolpropane and tolylene diisocyanate (a trade name “CORONATE L”manufactured by NIPPON POLYURETHANE INDUSTRY CO., LTD. was used) as anisocyanate-based crosslinking agent in an amount of 0.2 part based on100 parts of the acrylic polymer (in terms of solids content, and thesame shall apply hereinbelow). As such, a pressure-sensitive adhesivecomposition according to Example 10 was prepared.

Two sheets of the release liners (thickness: 38 μm) that had beenrelease-treated by a silicone-based releasing agent were prepared on thesurface of a polyethylene terephthalate (PET) film. The above-describedadhesive composition was applied onto the release surface (the surfacethat had been treated with the releasing agent) of the first releaseliner with an applicator, dried at 130° C. for 3 minutes to form apressure-sensitive adhesive layer having a thickness of 50 μm on therelease liner. Then, the pressure-sensitive adhesive layer was combinedwith the release surface of the second release liner. As such, apressure-sensitive adhesive sheet (a release liner-attached adhesivesheet) configured such that the both sides of the pressure-sensitiveadhesive layer were protected with the release liners was prepared. Thegel fraction of the pressure-sensitive adhesive as measured in the samemanner as in Example 1 was 49.2%.

Example 11

A pressure-sensitive adhesive composition was prepared in the samemanner as in Example 10, except that the monomer mixture of thecomposition shown in Table 3 (in the table, “HEA” representshydroxyethyl acrylate) was used, and that the amount of theisocyanate-based crosslinking agent to be added based on 100 parts ofthe acrylic polymers was 0.1 part, and a pressure-sensitive adhesivesheet was prepared using the composition. The gel fraction of thepressure-sensitive adhesive was 59.4%.

Example 12

A pressure-sensitive adhesive composition was prepared in the samemanner as in Example 10, except that the monomer mixture of thecomposition shown in Table 3 (in the table, CHMI″ representsN-cyclohexylmaleimid) was used, and that the amount of theisocyanate-based crosslinking agent to be added based on 100 parts ofthe acrylic polymers was 0.5 part, and a pressure-sensitive adhesivesheet was prepared using the composition. The gel fraction of thepressure-sensitive adhesive was 53.4%.

The evaluation tests as described above were carried out on thepressure-sensitive adhesive sheets prepared in Examples 9 to 12 (whichare provided with adhesive layers formed from the solvent-type adhesivecompositions). The results are shown in Table 3.

TABLE 3 Repelling resistance Peeling Monomer mixture (part) (mm)strength Holding 2EHA NVP CHMI HEAA HEA Corrosiveness 4 hr 7 hr (N/25mm) power Example 10 70 26 4 Absence 0.4 0.8 25.0 Good Example 11 70 264 Absence 1.5 2.9 23.5 Good Example 12 70 26 4 1 Absence 3.1 7.3 20.6Good

As shown in Table 3, according to the pressure-sensitive adhesive sheetshaving the copolymerization composition replacing HEA (Example 11) withan equivalent amount of HEAA (Example 10) in the monomer mixture, therepelling resistance was further improved (the total value of thefloating heights was decreased) while maintaining other characteristicsat least equivalently. Further, by the pressure-sensitive adhesivesheets having the copolymerization composition replacing CHMI (Example12) with an equivalent amount of HEAA (Example 10) in the monomermixture, an effect of improving the adhesive performances was seensimilarly. Moreover, all of the pressure-sensitive adhesive sheetsaccording to Examples 10 to 12 did not have corrosiveness on a metalsurface, and showed sufficient holding power.

As shown above, specific examples of the present invention are describedin detail, these are only for the illustrative purpose, and do not limitthe scope of the claims. The technologies as described in the scope ofthe claims include various modifications and alterations of the specificexamples as described above.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

This application is based on Japanese Patent Applications (PatentApplication No. 2008-135518 filed on May 23, 2008, and PatentApplication No. 2008-198269 filed on Jul. 31, 2008), the entirety ofwhich is incorporated herein by way of reference.

As described above, a pressure-sensitive adhesive sheet provided with apressure-sensitive adhesive layer formed by using the pressure-sensitiveadhesive composition of the present invention exhibits good adhesiveperformances (repelling resistance, static load characteristics, and thelike) at a normal temperature, and further, it can be preferably used,for example, for affixation of an article having a metal surface, inaddition to an electronic part, as a bonding material having asuppressed property of corroding a metal surface. The pressure-sensitiveadhesive sheet can be preferably used for other applications in theembodiment for directly applying on a metal surface, for example, forthe applications such as conveyance, protection, decoration, and thelike of articles having a metal surface, making an advantage of such thecharacteristics. The pressure-sensitive adhesive composition of thepresent invention is suitable as a pressure-sensitive adhesivecomposition used for a pressure-sensitive adhesive sheet to be attachedto a metal surface (typically for forming a pressure-sensitive adhesivelayer of the pressure-sensitive adhesive sheet). Further, thepressure-sensitive adhesive sheet provided by the present invention canbe suitably used in other application embodiments, without limitation tothe embodiments for directly attaching on a metal surface, since itexhibits good adhesive performances at a normal temperature as describedabove.

What is claimed is:
 1. A pressure-sensitive adhesive compositioncomprising, as a main ingredient, a monomer mixture or an acryliccopolymer material obtained by at least partially polymerizing themonomer mixture, said monomer mixture comprising: at least one monomer(monomer m1) selected from alkyl (meth)acrylates represented by thefollowing formula (I) in an amount of 50 to 85% by weight based on atotal amount of the monomer mixture:CH₂C(R¹)COOR²  (I) wherein R¹ is a hydrogen atom or a methyl group, andR² is an alkyl group having 1 to 20 carbon atoms,N-(2-hydroxyethyl)acrylamide (monomer m2) in an amount of 2 to 12% byweight based on the total amount of the monomer mixture, andN-vinyl-2-pyrrolidone (monomer 3), in an amount of 10 to 40% by weightbased on the total amount of the monomer mixture; and said monomermixture comprising substantially no carboxyl group-containing monomer.2. The composition according to claim 1, wherein the monomer m1, themonomer m2 and the monomer m3 are contained in a total amount of 90% byweight or more based on the total amount of the monomer mixture.
 3. Thecomposition according to claim 1, wherein the monomer mixture has such amonomer composition that the acrylic copolymer, obtained by polymerizingthe monomer mixture, has a glass transition temperature (Tg) of −10° C.or lower.
 4. The composition according to claim 1, which is used for apressure-sensitive adhesive sheet to be directly attached to a metalsurface.
 5. A pressure-sensitive adhesive sheet comprising apressure-sensitive adhesive layer formed by using the compositionaccording to claim
 1. 6. A method for producing a pressure-sensitiveadhesive sheet, said method comprising: preparing a pressure-sensitiveadhesive composition comprising a monomer mixture or an acryliccopolymer material obtained by partially polymerizing the monomermixture, and 0.01 to 2 parts by weight of a photopolymerizationinitiator based on 100 parts by weight of the monomer mixture, saidmonomer mixture comprising: at least one monomer (monomer m1) selectedfrom alkyl (meth)acrylates represented by the following formula (I) inan amount of 50 to 85% by weight based on the total amount of themonomer mixture:CH₂═C(R¹)COOR²  (I) wherein R¹ is a hydrogen atom or a methyl group, andR² is an alkyl group having 1 to 20 carbon atoms,N-(2-hydroxyethyl)acrylamide (monomer m2) in an amount of 2 to 12% byweight based on the total amount of the monomer mixture, andN-vinyl-2-pyrrolidone (monomer 3), in an amount of 10 to 40% by weightbased on the total amount of the monomer mixture, and said monomermixture comprising substantially no carboxyl group-containing monomer;applying the pressure-sensitive adhesive composition to a support; andcuring the applied composition by irradiating the applied compositionwith light to thereby form a pressure-sensitive adhesive layer.